Solar System's Moons May Have Emerged from Long-Gone Planetary Rings

"If you wish to make an apple pie from scratch," as Carl Sagan once said, "you must first invent the universe." And if you wish to make a moon from scratch, according to new research, you must first create planets with rings (after inventing the universe, of course).

Earth’s moon may have emerged from a long-vanished ring system, much like the rings still encircling Saturn – and the same goes for many of the satellites orbiting the other planets. The bulk of the solar system’s regular satellites—those moons that stick close to their planets in roughly equatorial orbits—formed this way, rather than taking shape simultaneously with the planets as a direct result of planet formation, French astrophysicists have concluded. The researchers reported their findings in the November 30 issue of Science.

“It’s fundamentally the same process that gave birth to the moon and to the satellites of the giant planets, and that’s the spreading of rings,” says astrophysicist Aurélien Crida of the University of Nice–Sophia Antipolis and the Observatory of Côte d’Azur in France, who co-authored the study with Sébastien Charnoz of the University of Paris–Diderot.

Through theoretical modeling, the researchers found that the moon-formation action begins at the edge of a planetary ring, where a satellite can take shape without being shredded by the gravitational pull of the planet. There, moonlets coagulate from the ring material before migrating outward. As the ring system spits out moonlet after moonlet, the small objects merge to form larger moons, which may merge in turn as they spiral outward from the planet.

The idea of a moonlet assembly line differs from the standard conception of satellite birth, in which moons condense along with their host planet from a swirling cloud of dust and gas, much like the planets themselves are thought to have taken shape around the nascent sun. The solar-system-in-miniature concept seems to work well for the largest moons, such as Jupiter’s four so-called Galilean satellites, but the retinue of smaller moons circling the other giant planets “have so far been considered a by-product,” Crida says.

The new hypothesis seems to explain a key commonality among the regular satellites of Saturn, Uranus and Neptune – namely, that moons farther from their respective planets tend to have larger masses than their closer-in neighbors. Like a snowball rolling downhill, the coalescing moons would grow larger and larger as they drift farther from the planet and its rings, undergoing progressively more mergers along the way. The end result is a neatly ordered satellite system, with small moons on the inside built from few moonlets and large moons farther out built from numerous moonlets.

“I think the best thing about this work is that they explain this link between the mass of the moon and the orbital distance, which was known before but not understood,” says planetary scientist David Nesvorny of the Southwest Research Institute in Boulder, Colo., who did not contribute to the new research. “If you had asked me a few years ago, I would think of our moon’s formation and the formation of the satellites of the outer planets differently,” he adds. “This theory puts things on common ground.”

Planetary scientists generally accept that a giant impact into the newly formed Earth ejected a huge cloud of material that became the moon. In Crida and Charnoz’s conception, that ejecta first flattened into a ring around the planet, which then spread out and coagulated into the moon. But unlike Saturn’s ring, which would have leaked out numerous moonlets to form several moons, Earth’s relatively massive ring would have poured all its material into one large satellite before dissipating. “It spreads very fast,” Crida says of Earth’s hypothesized ring. “And if it spreads fast, only one moon has time to form.”

But the new hypothesis is not without its problems. First, it does not seem to apply to the satellites of Jupiter, which do not obey the same mass-to-distance correlation of the other moon systems. Crida notes that Jupiter was the first planet to form and may have coalesced under different conditions. “I was a bit disappointed to see that Jupiter did not fit the same distribution, but not too surprised,” he says. And then there is the obvious question: if extensive, Saturn-like ring systems once adorned Neptune and Uranus, where are they now? “It’s not that easy to remove them over time,” Nesvorny says. “There is a link that’s missing that needs to be understood.”

The French researchers concede that the fate of the rings is an open question. “I don’t know why Uranus and Neptune don’t have the rings anymore,” Crida says. “We have a few ideas, but nothing too convincing. Some people can see that as a weakness of our mechanism, because indeed where are the rings now? But I think we can find good reasons for the disappearance of the rings, and the satellites remain as the smoking gun."